Kinetically Controlled Synthesis of Highly Emissive Au18SG14 Clusters and Their Phase Transfer: Tips and Tricks

Glutathione (GSH) protected gold nanoclusters (AunSGm NCs) have been attractive because of their novel properties such as enhanced luminescence and band gap tunability at their quantum confinement region (below ∼2 nm). Initial synthetic routes of mixed-size clusters and size-based separation techniques had latter evolved toward atomically precise nanoclusters via thermodynamic and kinetic control routes. One such exemplary synthesis taking the advantages of a kinetically controlled approach is producing highly red-emissive Au18SG14 NCs (where SG = thiolate of glutathione), thanks to the slow reduction kinetics provided by the mild reducing agent NaBH3CN. Despite the developments in the direct synthesis of Au18SG14, several meticulous reaction conditions still need to be understood for the highly adaptable synthesis of atomically pure NCs irrespective of the laboratory conditions. Herein, we have systematically studied a series of reaction steps involved in this kinetically controlled approach starting from the role of the antisolvent, formation of precursors to Au-SG thiolates, growth of Au-SG thiolates as a function of aging time, and exploring an optimal reaction temperature to optimize the desired nucleation under slow reduction kinetics. The crucial parameters derived in our studies guide the successful and large-scale production of Au18SG14 at any laboratory condition. Next, we investigated the effect of pH on the NCs to study the stability and the best suitable condition for the phase transfer of Au18SG14 clusters. The commonly implemented method of phase transfer at the basic conditions (pH > 9) is not successful in this case. However, we developed a feasible method for the phase transfer by diluting the aqueous NC solution to enhance the negative charges on the NCs’ surface by increasing the degree of dissociation at the carboxylic acid group. It is interesting to note that after the phase transfer, the Au18SG14-TOA NCs in toluene as well as in other organic solvents exhibited enhanced luminescence quantum yields from 9 to 3 times and increased average photoluminescence lifetimes by 1.5–2.5 times, respectively.

S2 weight of 307) were purchased from Sigma-Aldrich. Deionized water (18.2 MΩ cm) was used in our experiments.

Synthesis of Au18SG14:
The original synthesis was reported by Atanu et al. and modified the reaction temperature as per our required study. In typically synthesis of Au18SG14 clusters, about 150 mg of HAuCl4.3H2O (solid) was added to 1 mL MeOH, following the addition of 2 mL distilled water to form HAuCl4 (1MeOH: 2H2O) solutions. 300 mg GSH was added to above prepared HAuCl4 solutions. This solution was subjected to ultrasonication for several minutes to transform the color of the solution from yellow to colorless. Next, the solution was diluted by addition of 45 mL of MeOH. The stirring time is crucial for the high yield synthesis of Au18SG14, which is around 5 to 10 mins (further discussions are given in the main text). In order to study the effect of reaction temperature, two such solutions were placed at two different reaction temperatures such as 17 ℃ and 37 ℃, respectively. After that 4.5 mL of aqueous NaBH3CN solution was added stepwise by 1 mL pipette (5-6 times) and continued the stirring for 1 hour. Next, these formed nanoclusters were subjected to purification process.
The Au18SG14 NCs were precipitated out from the solution through centrifugation process. The precipitate is further washed with MeOH for 2-3 times to remove the impurities such as excessive glutathione and the remanent species from reducing agent. The NC precipitate was re-dissolved in water to produce a turbid solution, which was subjected to centrifugation to remove majority of insoluble thiolates as a pellet, but minute amounts of free flow thiolate species could be removed by filtering with filter the paper having 0.22 µm pores. Finally, we get the Au18SG14 aqueous solution and keep it in the fridge to store for long period of time (≈ 15 days). And we can also get the Au18SG14 powder through freeze dried Au18SG14 aqueous solution.
Methods: SHIMADZU UV-vis 1900 spectrometer was used for the measurements. UV Spectra were typically measured in the range of 200-1100 nm. A diluted solutions were spotted S3 on carbon-coated copper grid and was dried in air conditions. Images were collected at 120 keV, which reduces beam-induced damage to the clusters. The photoluminescence spectra and lifetime of the Au18(SG)14 and TOA-paired Au18(SG)14 clusters were recorded by using Fluo Time 300. The Zetasizer Nano ZSE was used for testing the Au-SG complexes size and distribution.
Phase transfer of highly luminescent Au18SG14-TOA Clusters: The two methods were used for the phase transfer of Au18SG14 clusters. In the typically phase transfer, two solutions are mixed, namely i) 20 mg TOAB was dissolved in 10 mL toluene, and ii) 25 mL of aqueous Au18SG14 solution (half the amount of one synthetic batch) whose pH is adjusted to 9 by the addition of NaOH. The toluene solutions which contain TOAB were added to Au18SG14 solutions and stirred it for several minutes. A colorless toluene solution changes to reddish brown indicates the clusters undergone the phase transfer. In another phase transfer method, initially, 25 mg TOAB was dissolved in 20 mL toluene, the 20 mL Au18SG14 solutions were added to a beaker, then diluted the solution by 8 times (≈ 160 mL). Next, the toluene solutions which contain TOAB were added to the Au18SG14 solutions and vigorously stirred for several minutes results the phase transfer of clusters from aqueous to organic medium.